Fracturing fluids comprising gels and crosslinked gels are widely used to fracture subterranean formations so as to allow the flow of fluids such as oil and gas therethrough, and make these hydrocarbon materials accessible to pumping. Gels, which can carry particulate propping agents to keep the fracture open, are preferred fracturing fluids. Crosslinked gels are preferred as having better pumping characteristics. In general such crosslinked polymers comprise an aqueous liquid, a gelling agent and a crosslinking compound. The gelling agents in general are hydratable polysaccharides having molecular weights greater than about 100,000. These include galactomannan gums, glucomannan gums, and cellulose derivatives. Among the most widely used gelling agents in the industry at the present are hydroxypropylguar gums (HPG), hydroxyethycellulose (HEC), and the carboxymethyl-substituted derivatives of each, carboxymethylhydroxypropylguar (CMHPG) and carboxymethylhydroxyethylcellulose (CMHEC). Crosslinking agents include compositions, preferably organic compositions, containing polyvalent metal ions, especially metal ions capable of +3 and +4 valent states, such as Al.sup.+3, Ti.sup.+4 and Zr.sup.+4. The pH affects the speed of hydration of the gelling agent as well as the speed of crosslinking, and pH-adjusting agents are often included in the fracturing fluid mix. Commonly used pH-adjusting agents are hydrochloric acid, fumaric acid, and phthalic acid, as well as potassium biphthalate, sodium hydrogen furmarate and sodium dihydrogen citrate, to name only a few.
It is desirable in using crosslinked gels for fracturing subterranean formations that the crosslinking reaction proceed while the gel is traversing the tubular goods toward the formation. If crosslinking occurs either in or prior to entering the pumps, pumping difficulties may be encountered. If crosslinking does not occur before the gel reaches the formation, the gel will not have the viscosity required to place large quantities of proppant in the formation. Typically, gelling compositions hydrate rapidly. Because it is desirable for the gelling composition to be as completely hydrated a possible prior to addition of the crosslinking agent, prior art processes have relied on prehydration of the gelling composition in storage tanks prior to the addition of the crosslinking agent. Quality control of the final mix is sometimes difficult, and it often happens that injection pumps will be slightly out of adjustment for purposes of providing the proper flow rate for the amount of crosslinking agent being added to the gel as it is being pumped into the formation. With too much crosslinking agent, the composition gels too much or too fast for optimum flow into the formation, and with too little crosslinking agent, the composition will be too thin to advantageously effect fracturing. Adding the crosslinking agent to the gelling composition in the storage tanks would only exacerbate pumping and removal problems due to tremendous viscosity increases. Further, if the crosslinking agent becomes active before the gelling composition is completely hydrated, further hydration is essentially halted and peak viscosity will never be reached, resulting in an inferior fluid.
It is desirable that both hydration of the gelling composition and crosslinking be accomplished during the period the fracturing fluid is traveling down the well bore, so as to allow the fluid to be easily pumped, yet be viscous enough in the formation to accomplish the desired fracturing and proppant transport. It would be highly advantageous to provide prepackaged mixtures of all the dry ingredients required for the crosslinked polymer, including the crosslinking agent, in the appropriate proportions, so that it is necessary only to add these dry ingredients to the water or aqueous liquid being pumped down the well into the formation to enable mixing and pumping on a continuous basis without the need for pregelling tanks.